Detector-probe technique

Detector-probe technique It can find the location of a leak when the tracer gas is on the inside of the piece in question. This technique is time-consuming. Because the probe is collecting both air and helium, it is about 10% as sensitive as the tracer-probe technique. It cannot tell you the size of the leak. 

Some of the other leak detection methods explained earlier in this section are based on these techniques. Methods such as bagging parts of a vacuum system and filling the bagged parts with a probe gas are similar to the outside-in technique. However, it is not possible to quantify a leak with this approach. Alternatively, filling a container with gas and either submerging it in a liquid or covering it with a bubbling solution is similar to the detector-probe technique. 

Fig. 7.59 In the detector-probe technique, the tested piece is filled with helium and the sniffer probe sniffs the areas in question to detect leaks. Always sniff from bottom to top.

Most helium leak detectors will not operate with pressures above lO"4 torr to 10 5 torr. At these greater pressures, the main element to the mass spectrometer will bum out. Fortunately most, if not all, helium leak detectors have various safety check mechanisms that automatically shut off the current to the main filament if the pressure goes above a set limit. So, you must depend on alternate leak detection methods, or use the detector-probe technique to discover large leaks. Once large leaks have been discovered and closed, you can concentrate on the smaller leaks that can be found with the tracer-probe technique. 

Aside from permeability and absorbency complications, other universal concerns of helium leak detection are factors such as source operating pressure, spraying patterns (for tracer-probe technique), response time, clean-up time, and cold trap usage. Pump use and general helium leak detector maintenance operations are also fairly universal. 

The source operating pressure is the vacuum necessary to operate the leak detection device. This pressure is not specific, rather it is a pressure range within the leak detector which works. Optimistically, we want the helium leak detector, and the system to which it is connected, to have the greatest possible vacuum. This gives the tracer-probe technique the maximum sensitivity with the quickest response time. As an added benefit, when one is operating at a very high vacuum,... 

Fig. 7.65 A suggested pattern for spraying helium on a vacuum system when using the tracer-probe technique. The pattern is a compromise between proximity to the leak detector and spraying high before low areas.

When a very high sensitivity is needed the pump/probe technique using photo-ionization was employed [46J. The atoms were excited by the pump beam and then a delayed probe beam was applied to ionize them from the excited state. The ion signal versus delay time is detected. Advantages of this technique are a high sensitivity and a time resolution limited only by the duration of the pump and probe pulses, but not by the time resolution of the detector. 

Further progress in experimental techniques will allow lifetime measurements in neutral and singly ionized atoms to be extended to even shorter wavelengths and lifetimes. Any free neutral and singly ionized atom can be produced in a lasergenerated plasma. For short-lived states, the pump-probe technique allows to overcome the problems with detector response time. A distributed feedback laser pumped by picosecond pulses from a mode-locked NdrYAG laser can serve as a light source. 

The pulse-probe technique can be extended to multiwavelength detection by using the ultrafast laser pulse to generate a white-light continuum probe, which can be dispersed with a spectrograph across a diode array or CCD detector after traversing the sample. Due to lower probe intensity, Cerenkov emission from the sample would be expected to be more of a complication in this case, but the correction methods developed for stroboscopic Cerenkov detection would also work here. 

The time resolution of the point-by-point and step-scan FTIR approaches is limited by the rise time of the fast IR detector used in the experiment (ca. 10 ns). However, many photochemical and photophysical events take place on the subnanosecond timescale, which require a faster technique. Ultrafast IR spectroscopy is a variant of the pump-probe technique, where time resolution is achieved by spatially delaying the probe pulse with respect to the pump pulse (Figure 5). 

During the last 15 years, several technical developments in the light source and detector made time-resolved Raman spectroscopy an important branch of time-resolved surface spectroscopy [60]. It should be emphasized that there are two kinds of time-resolved studies. The first kind of measurement is triggered by a certain surface process (reaction) named as a single shot experiment and the second one is a pump and probe technique. 

Today two important techniques are used to avoid secondary collisions. The first technique is the molecular beam method, in which the density i kept so low that no collisions occur before the products reach the detector (for example a mass spectrometer). The second technique, that became more popular in the recent years, is based on time resolution. In this "pump and probe" technique, two short laser pulses... 

For measurements of very fast relaxation processes that demand a time resolution below 10 s most detectors (except the streak camera) are not fast enough. Here the pump-and-probe technique is the best choice. It is based on the following principle shown in Fig. 6.96. 

The time resolution of the pump-and-probe technique is not limited by the rise time of the detectors. It can therefore be used in the pico- and femto-second range (Sect. 6.4) and is particularly advantageous for the investigation of ultrafast relaxation phenomena, such as collisional relaxation in liquids and solids [1042, 1043]. It is also useful for the detailed real-time study of the formation and dissociation of molecules where the collision partners are observed during the short time interval when forming or breaking a chemical bond [1044]. 

Figure 5 Diagram of pump-probe technique. Since the chemical system is only observed while the probe pulse is present, the time resolution of the detector doesn t matter. The detector could be a photodetector or an upconversion detector. The pump beam could be light or electrons.

Electroanalytical metiiods Molecularly designed electrodes, speciation Microelectrodes, biological probes Electrochemical detectors, combination techniques... 

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